Plastic materials are lightweight, highly tough, and easy to dye, and therefore are in wide use for various optical materials, especially for glass lenses recently. Properties required for optical materials, especially glass lenses are as follows. Physical properties required are low specific gravity, high transparency, low yellow index, high thermal resistance, high strength and the like. Optical properties required are high refractive index and high Abbe number. A high refractive index can thin the lens, and a high Abbe number reduces the chromatic aberration of the lens. However, as the refractive index increases, the Abbe number decreases. Studies are being made for improving both of the refractive index and the Abbe number. The most representative method developed so far is a method using an episulfide compound described in Japanese Patent No. 3491660.
A lens thinned as a result of an increase in the refractive index is desired to have a higher strength than conventional lenses from the viewpoint of safety and the like. There are roughly three types of strength required for the lens. First is strength against dropping. A lens needs to have a sufficient strength not to be easily destroyed when colliding against an object or when being dropped. Second is strength against drilling. A lens should not be chipped or cracked when a hole is made in the lens by a drill; namely, a lens needs to have a good drilling processability. When a so-called two point frame is attached to the lens, holes need to be made in the lens. The lens needs to have a sufficient strength not to be chipped or cracked by this procedure. Third is a tensile strength. In order to be durable against use with a two point frame, a lens needs to have a sufficient strength not to be destroyed even when the frame is bent.
Thermal resistance is also required. As a standard practice, a plastic lens is provided with hard coat in order to protect the surface thereof. Since the lens is heated when being provided with hard coat, the lens needs to be thermally resistant.
Conventionally, various attempts have been made to strengthen materials which use an episulfide compound as described in Japanese Patent No. 3541707, Japanese Laid-Open Patent Publication No. 2001-131257, Japanese Patent No. 3562579, Japanese Laid-Open Patent Publication No. 11-352302, Japanese Patent No. 3642973, Japanese Laid-Open Patent Publication No. 2005-272788, and Japanese Patent No. 3706036. These attempts end in evaluating the results in only some strength tests, and anything which provides a satisfying result in all the strength tests has not been provided. Needless to say, further improvement in the strength is also desired. In some cases, when it is attempted to obtain a high strength lens, thermal resistance is decreased (Japanese Patent No. 3541707, Japanese Laid-Open Patent Publication No. 2001-131257, and Japanese Patent No. 3562579). Providing both of strength and thermal resistance is indispensable but difficult. In some other cases, cured materials are cloudy, or lenses are not sufficiently cured to be practically used for glasses (Japanese Laid-Open Patent Publication No. 11-352302, Japanese Patent No. 3642973, and Japanese Laid-Open Patent Publication No. 2005-272788). In another case, a material which is difficult to be synthesized is used and such a material is thus difficult to be put into practice (Japanese Patent No. 3706036).
Accordingly, although a lens using an episulfide compound has a high refractive index and a high Abbe number, a lens having a higher strength and a higher thermal resistance has been desired to be developed.